1. Field of the Invention
This invention relates to metal halogen cells having an aqueous solution of a metal halide as the electrolyte. In particular, the present invention relates to improved cells and batteries employing a zinc or cadmium anode, a bromine cathode and an aqueous metal halide electrolyte in which the metal of the metal halide is the same as the metal of the anode.
2. The Prior Art
As is well known in the art, electrochemical cells have been proposed which have an electrode with a high positive oxidizing potential and another electrode with a strong negative or reducing potential. Typical of such cells is the metal halogen cell in which the anode material most commonly employed is zinc and the most commonly employed cathodic halogen is bromine. Among the advantages of such cells is their extremely high theoretical energy density. For example, a zinc bromine cell has a theoretical energy density of 200 wh/lb (i.e. watt hours per pound) and an electric potential of about 1.85 volts per cell.
In such a cell the surface of the metal anode, for example, zinc, oxidizes thereby undergoing a positive increase in valence. As a result thereof, zinc atoms are converted to zinc ions which enter the electrolyte according to the equation: EQU Zn .fwdarw. Zn.sup.++ +2e
The chemical reaction occurring at the cathode is expressed by the following equation: EQU Br.sub.2 + 2e .fwdarw. Br.sup.-
Thus, the overall chemical reaction can be written as follows: EQU Zn + Br.sub.2 .revreaction. Zn.sup.++ + 2Br.sup.-
The arrow to the right indicates the direction of the chemical reaction occurring during cell discharge and the arrow to the left indicates the chemical reaction occurring during charging of the cell.
The electrochemical cells of the foregoing type are known to suffer from a number of disadvantages. Most of these disadvantages are associated with side reactions which may occur in such cells. For example, during the charging process free bromine is produced in the cell. This free bromine is available for chemical reaction with the metal anode thereby resulting in auto discharge of the cell. Additionally, there is a tendency for hydrogen gas to be generated when considerable amounts of free bromine are present in the aqueous phase.
The art is replete with efforts on the part of many inventors to overcome the above-mentioned disadvantages. In U.S. Pat. No. 2,566,114, for example, the use of tetraethyl and tetramethyl ammonium bromide for combining with bromine generated during charging of the cell is disclosed. The tetramethyl ammonium salt is added to the powdered carbon surrounding the cathode.
In U.S. Pat. No. 3,738,870 the use of a solid mixture of alkyl ammonium perchlorate and conductive materials such as graphite to form solid addition products with halogen released during charging of such cells is disclosed.
In U.S. Pat. No. 3,811,945 the use of certain alkyl ammonium perchlorates, diamine bromides and diamine perchlorates which are capable of forming solid addition products with cathodic bromine and which are substantially insoluble in water is disclosed.
In contrast to those references which suggest forming solid addition products with bromine, U.S. Pat. No. 3,816,177 discloses the use of a quaternary ammonium halide and a depolarizer in the electrolyte. The depolarizer functions as an organic complexing solvent which dissolves in water and is not reactive toward the halogen or halogens in the cell. The function of the depolarizer apparently is to form water insoluble complex in the presence of quaternary ammonium halides.
As will be readily appreciated, however, even with the use of the aforementioned complexing techniques self-discharge of metal halogen cells will not be totally eliminated since some of the cathodic bromine will remain in the aqueous phase notwithstanding the use of these complexing agents. Indeed, the presence of some halogen is desirable particularly when current is being withdrawn from the cell.
Thus, while the many references cited above show a continuing effort on the part of numerous inventors to overcome the disadvantages associated with metal halogen cells of the type referred to herein, the methods proposed have not adequately overcome the problem encountered in such systems. Consequently, there remains a need for more effective methods for preventing loss of cell capacity and aqueous metal halogen cells.